As is well known, photolithography is a technique which in recent years has facilitated more effective and inexpensive manufacture of semiconductor devices, such as transistors and integrated circuits. Such semiconductor devices are the building blocks of virtually all consumer, industrial, and military electronic apparatus today, including computers, calculators, automated equipment, and televisions, which collectively have done much to improve the quality of life in our country in recent years.
In the practice of photolithography, a pattern in an optical mask, which corresponds to the features of the integrated circuit to be manufactured, is imaged onto a semiconductor wafer with ultraviolet light (UV). The wafer is coated with a UV-sensitive photoresist, which changes chemically during exposure to the UV light over areas determined by the pattern in the mask. After exposure, the photoresist is developed, and the semiconductor wafer is further processed by etching away areas determined by the imaged pattern and by depositing impurities. The process may be repeated on the wafer until the desired transistor device or integrated circuit is fabricated.
The light source which is used to provide the ultraviolet light for conventional photolithography is typically an electrode arc lamp with a mercury fill in which the radiation is provided by an arc discharge which occurs between two electrodes in the lamp envelope. For exposure of conventional photoresists, the lamp provides radiation at the conventional UV wavelength of 260-460 nm.
A goal in the fabrication of integrated circuit is to reduce the size of circuit features as much as possible so that more or faster circuit components may be included in a single integrated circuit chip of a given size. Thus, it is generally recognized that a computer of today which is small enough to be placed on a table has as much computational power as a room-size computer of a generation ago, with a corresponding reduction in cost. This tremendous reduction in size and cost has largely been made possible by the ascendancy of integrated circuit technology, including the photolithographic techniques discussed above, which have permitted the printing of microscopic circuit features.
It is of course desired to continue in the same direction to provide integrated circuits having even greater component densities. However, it has been discovered that as the resolution of imaged lines approaches 1 .mu.m in width, the conventional UV wavelengths are too long, and results in diffraction effects which impair effective imaging. This can be intuitively understood when it is appreciated that at such narrow line widths, the mask slits themselves are closer in dimension to the wavelength of the light being used, which influences the behavior of the light as it passes through the slits.
To solve this problem it is necessary to use an imaging medium having a shorter wavelength than conventional ultraviolet. While several approaches have been proposed, including the exploratory technologies of x-ray and e beam, the most promising of these is use of deep ultraviolet light (190-260 nm). It is preferred to the exploratory technologies, since a substantial part of the deep UV system configuration, including for example, masks and alignment apparatus are already available from conventional ultraviolet.
Also, apart from its use to print high resolution lines of submicrometer width, deep UV may be advantageously used to improve imaging when printing lines of conventional resolution. Thus, as known to those skilled in the art, use of shorter wavelengths results in a greater depth of focus at the wafer and maximizes the probability of sharp printing even when the mask and wafer are not precisely positioned.
For the above reasons, for the last several years, substantial effort has gone into developing a successful deep UV photolithography system. A suitable deep UV photoresist known as polymethyl methacrylate (PMMA) has been developed and is in use. However, the one problem which has eluded solution, and which has kept deep UV from realizing its potential for providing integrated circuits of greater packing density, has been the lack of a satisfactory light source. The limitations of existing light sources for deep UV photolithography have been well documented, e.g., see "Optical Lithography in the 1 .mu.m Limit" by Daryl Ann Doane, Solid State Technology, August 1980, Pp. 101-114 and "A Practical Multilayer Resist Process for 1 .mu.m Lines" by Batchelder et al, Semiconductor International, April, 1981, Pp. 214-218.
The primary problem has been the inability of workers in the art to provide a light source having sufficient brightness in the deep UV part of the spectrum to effect rapid on-line exposure of deep UV photoresists. This has resulted in unacceptably long processing times and consequent low yield per unit time of completed semiconductor devices. Thus, presently the source which is most widely used for deep UV applications converts less than 2% of its input power to output radiation in the deep UV part of the spectrum. Additionally, this source has a relatively short operating lifetime of only about 100 hours, which has resulted in frequent downtime for the purpose of changing lamps. Additionally, a number of other types of deep UV sources have been tried, but these have also resulted in relatively low deep UV output, and typically have been hampered by other problems and disadvantages, such as nonuniform light output, a spectral output which deteriorates rapidly with age, and the necessity for critical positioning.
The literature discloses that the following light sources have been used or considered for deep UV photolithography:
(1) The xenon-mercury (Xe-Hg) compact arc lamp is the primary source which has been used, and is a high pressure electrode arc lamp. It is similar to the mercury compact arc lamp which is used for conventional UV photolithography with the addition of xenon in the fill to alter the spectrum towards the deep UV. However, even with such addition, only a very small portion of the lamp output falls within the desired 190-260 nm range. Thus, the lamp must be run at very high power levels to extract what deep UV is possible, but even so, exposure time is longer than desired. Additionally, the high power levels at which the lamp must be operated contribute to rapid aging, resulting in degradation of the spectrum produced and the necessity of too-frequent bulb replacement.
(2) The pulsed xenon lamp is a low-medium pressure arc lamp driven by short, high energy pulses, which delivers a continuum from 200-315 nm, and has about 6% of its output between 200-260 nm. Relatively non-uniform light output and production of radio frequency interference have limited application of this lamp.
(3) The deuterium lamp produces a continum in the region of 200-315 nm, but it has been found that output levels are too low for practical application.
(4) The pulsed mercury lamp is a high pressure arc lamp which is driven with short, high energy pulses. It provides a continuum between 200 and 300 nm, but lacks repeatability and suffers from short life.
(5) Doped lamps are typically compact arc sources with enhanced spectral emissions produced by doping lamp materials during manufacture. It has been found that this type of lamp tends to be inconsistent in output, and has a short life span.
(6) Low power sources such as flourescent or germicidal lamps which have a significant part of their spectral output in the deep UV have been considered. However, the output power and brightness, i.e., (output power)/surface area), of these sources is not sufficient for photoresist exposure.
In addition to the above considerations, it is important to appreciate that all of the prior sources which have been used are lamps which include electrodes, and which emit light by generating an arc discharge between such electrodes. The generated arc is typically longer than it is wide, and is a non-uniform unstable emitter of radiation. The problems which are engendered by the existence of the arc discussed in Lovering U.S. Pat. No. 3,569,083, and for example have made mandatory the use of an optical integrator to uniformly redistribute the light before it is incident on the wafer. Further, since the arc is treated optically as a point source, the arc lamp must be positioned within extremely critical tolerances for suitable imaging.
It is the purpose of the present invention to provide a method and apparatus for performing deep UV photolithography using an improved light source, and more particularly a bright light source which has a substantial output in the deep UV part of the spectrum, resulting in much more rapid exposure of deep UV photoresists than in the prior art. Thus, it is anticipated that with the use of the present invention, the time necessary for such exposure is only a fifth to a tenth of that required by prior art systems. Additionally, the invention provides a light source having substantially longer lifetime than the other sources discussed, and overcomes others of the problems and disadvantages associated with such sources. More specifically, the objects of the invention are as follows:
It is an object of the invention to provide a method and apparatus for performing deep UV photolithography utilizing a light source which has a greater output in the deep UV part of the spectrum than sources heretofore used.
It is a further object of the invention to provide a method and apparatus which is capable of printing narrow lines on semiconductor wafers with shorter exposure times and consequently greater speed than has heretofore been possible.
It is still a further object of the invention to provide a method and apparatus for performing deep UV photolithography utilizing a light source which is more efficient than those heretofore used.
It is still a further object of the invention to provide a method and apparatus which utilizes a deep UV source which has a more uniform output than prior sources used, and which permits elimination of the optical integrating means typically used in the optical trains of such systems.
It is still a further object of the invention to provide a method and apparatus in which lamp bulbs fail less frequently than in prior art systems, thereby requiring less down time for bulb replacement.
It is still a further object of the invention to provide a method and apparatus which does not require critical source placement.
It is still a further object of the invention to provide a method and apparatus which utilizes a deep UV source having a spectrum which does not deteriorate greatly with age.